Administration of the MapR Database is done primarily via the command line (maprcli) or with the MapR Control System (MCS). Regardless of whether the MapR Database table is used for binary files or JSON documents, the same types of commands are used with slightly different parameter options. MapR Database administration is associated with tables, columns and column families, and table regions.

The MapR Data Access Gateway is a service that acts as a proxy and gateway for translating requests between lightweight client applications and the MapR cluster. This section describes considerations when upgrading the service, how to modify configuration settings, and how to administer and manage the service.

Managing Hard Links

A hard link is a directory entry that associates a name with a file (or physical data) on
the file system. Hard links allow multiple names to be associated with the same data (and
associated inode) from within or outside of a directory. Every time a hard link is created,
a directory entry is created and the inode (associated with the directory entry) remains the
same across all hard links associated with that data. That is, all names associated with the
data point to the same inode.

The following diagram illustrates the hard link semantics. Here, directory entries in dirA,
dirB, and dirC for file names name1, name2, and name3 respectively all point to the inode 12345,
which contains metadata including the text in the file and a count of the number of hard links
to the file (or physical data).

In contrast, when a symbolic link is created, a new inode is created and the text part of
the inode (associated with the symlink file) contains the path to the actual file. The following
diagram illustrates the symbolic link semantics. Here, the directory entry in dirA for file name,
name1, is associated with inode 12345, which contains the text in the file. The directory entry
in dirB for symbolic link file, name2, is associated with inode 13579, which contains the path
to file in dirA (/dirA/name1). Deleting the file, name1, in dirA will result in the symlink file
in dirB, name2, pointing to stale content, which in turn will return errors when accessed.

Hard links can be created on regular files, symlink files, device files, and tables.

Limitations

Hard links cannot be created on directories.

Hard links cannot be created across volumes or clusters. Instead, use symbolic link to
link to a file on a different volume.

Hard links within a volume are carried over to mirror volumes and volume snapshots.
During cross-mirroring, there will be an error if support for hardlinks is not
enabled on both the clusters.

The hadoop
distcp command cannot be used for creating and preserving copies of
hard links.

The maximum number of hard links is constrained by the integer width (32 bits), which
means the maximum number possible for a file is 232.

Usage

Any user with access to the directory can create a hard link to any file under that directory.
To create hard links, the user must have write permissions on the directory and execute permissions
(to do the lookup for the path) on the target file. To read or modify the file, the user must have
read or write permissions respectively on the file.

Errors

For information on the type of failures and errors, refer to the man page. In
addition, please note that the EXDEV error is returned if command is run to create
cross-volume or cross-cluster hard links.